Conventionally, a flash memory having a memory cell array configured by NAND connection or NOR connection of memory cells each having a floating gate structure has been well-known as an electrically rewritable nonvolatile memory. Also, a ferroelectric memory has been known as a memory that is nonvolatile and can be randomly accessed at a high speed.
Meanwhile, a variable resistance memory using a variable resistance element in each memory cell has been proposed as a technique for progressing miniaturization of memory cells. Known variable resistance elements include a phase change memory element, an MRAM element, a polymer ferroelectric RAM (PFRAM) memory element, a ReRAM element, etc. The phase change memory element changes its resistance value in accordance with state changes of a chalcogenide compound between a crystalline state and an amorphous state. The MRAM element uses resistance changes caused by a tunnel magnetoresistance effect. The PFRAM memory element has a resistance element made of a conductive polymer. The ReRAM element causes resistance changes in response to electric pulse application.
In the variable resistance memory, memory cells can be each configured by a series-connected circuit between a variable resistance element and a diode instead of a transistor. Therefore, it can be stacked easily. In addition, the variable resistance memory is advantageous in that it can be highly integrated by being formed in a three-dimensional structure.
Immediately after it is manufactured, the variable resistance element of the variable resistance memory has a very high resistance value and exists in a state that does not allow the resistance value to be easily changed. Hence, it is necessary to perform a forming operation that applies a high voltage (a forming voltage) to the variable resistance element. The forming operation generates a state that allows the resistance value of the variable resistance element to shift between a high resistance state and a low resistance state, and thereby enables the variable resistance element to function as a memory cell.
However, since a forming voltage is a high voltage, a leak current from the variable resistance element becomes large. Hence, there is a problem that a forming voltage equal to or higher than a certain value cannot be applied across both ends of the variable resistance element, and an accurate forming operation cannot be executed on the variable resistance element.